RF transistor package and mounting pad

ABSTRACT

An improved semiconductor package is provided wherein the mounting pad for the semiconductor is made from a material selected from the group consisting of aluminum nitride, diamond, alumina, and boron nitride.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention provides a novel mounting package for RF transistorsincluding a novel thermally conductive electrically insulating mountingpad, and relates to the semiconductor packaging industry.

2. Description of Related Art

The use of semiconductor devices for RF (radio or high frequency)applications has increased dramatically as heat dissipation, size,dependability, and other characteristics have made them particularlysuited for many applications. Semiconductor transistors are very smalland very difficult to use; therefore, they are generally incorporatedinto packages. Radio frequency packages are used for holdingsemiconductor components, particularly transistors, and for providingreadily available terminals for connection to other components.

In designing such packages, the tendency of semiconductors to generateheat must be considered. To this end, the semiconductor is frequentlymounted on a thermally conductive pad to act as a heat sink anddissipate heat generated by the transistor. However, semiconductors aresensitive to electrical energy, and therefore the thermally conductivepad should also be electrically insulating. Hence, a thermallyconducting, electrically insulating material is used for the mountingpad. Commonly, this material is beryllia (beryllium oxide--BeO),although alumina (aluminum oxide--Al₂ O₃) has occasionally been used.

Beryllia (BeO) has better thermal conductivity than Al₂ O₃, and istherefore more commonly used. However, care must be taken in handlingand processing BeO. BeO is highly toxic. It can be hazardous to thehuman respiratory system when in powder form. Therefore, care must beexercised if the BeO pad is to be machined or ground. Proper equipmentand safeguards are needed to insure that the BeO is handled safely andthat improper contact with humans is not made.

Another disadvantage of the prior art packages is that when BeO is used,a "thin" pad of BeO is employed. This pad is attached to the heat sinkthrough the use of a preform. The preform is generally made of gold andtin, or silver and copper. The preform is placed between the mountingpad and the heat sink and the structure is heated. This melts thepreform and brazes the pad to the heat sink. This creates twointerfaces: pad-to-preform, and preform-to-heat sink. These interfacesdecrease the thermal conductivity of the connection. Further, BeO has ahigh thermal conductivity up to about 200° C. to 250° C., but itsperformance decrease with increasing temperature.

The BeO pad used is generally about 0.040" to 0.060" thick. Suchthickness is required since the mechanical strength of BeO is low. Ifthe pad is not sufficiently thick, the pad may crack or split duringattachment, or later upon heating from use.

SUMMARY OF THE INVENTION

It has now been discovered that other thermally conductive, electricallyinsulating materials may be used as mounting pads for mountingtransistors and other semiconductors in packages. These materials may beapplied directly to the surface of the heat sink in very thin films, ormounted on the heat sink as thin slivers. This increases the thermalconductivity of the package and decreases the overall height of thefinished transistor package.

The materials which are useful for the present invention are; boronnitride (BN), diamond, aluminum nitride (AlN), and aluminum oxide/(Al₂O₃). The BN, AlN, and Al₂ O₃ may be deposited by plasma deposition. Thediamond thin coating may be deposited by vacuum deposition. Any of theabove materials may alternatively be mounted on the heat sink by placinga preform between the heat sink and a sliver of the material and heatingthe combination to braze the two together. These films or slivers can besubstantially thinner than the BeO pads of the prior art, since thematerials possess higher mechanical strength. Further, the films aredeposited directly on the heat sink, thus eliminating onematerial-to-material interface.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a three dimensional expanded view of a package of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to the FIGURE, package 10 of the present inventionincludes heat sink 20 with capacitor 30. Heat sink 20 is preferably madefrom tungsten and copper (W+Cu) which have qualities desirable for aheat sink. Heat sink 20 acts as a support for package 10. Capacitor 30is mounted directly on heat sink 20 and inside of the assembled package10.

Mounting pad 40 is attached to heat sink 20. Semiconductor 50 is thenattached to mounting pad 40. These attachments will be explained furtherpresently. Preform 60 is placed on heat sink 20, around capacitor 30,pad 40 and semiconductor 50. Lamina 70 and 80 are then placed on preform60. The lamina 70 and 80 act as spacers to allow proper connections tobe made to semiconductor 50. Lamina 70 and 80 are co-fired with leads 75(for connection to semiconductor 50 on the inside) already in place. Asecond preform 90 is placed on lamina 70 and 80, and lid 100 caps theentire package.

Lamina 70 and 80 are generally made from alumina and allow properpositioning of leads 75 with respect to semiconductor 50. Lid 100 isgenerally made from kovar, an iron-cobalt alloy. Preforms 60 and 90 aremade from gold and tin, or copper and silver. This allows lowtemperature brazing.

Mounting pad 40 may be made from any of several different materials andmay be a sliver of material or a very thin coated layer. Pad 40 may bemade of boron nitride, diamond, aluminum nitride, or alumina. Pad 40 isattached to heat sink 20 in different ways depending on the form of thepad.

A pad 40 made from diamond may take two different forms: a singlecrystal substrate, or a thin film. A single crystal substrate would befrom 3 to 4 mils (0.003"-0.004") thick. Such a single crystal substratewould be bonded to heat sink 20 through the use of a gold and tin, orcopper and silver preform. The preform is placed between the sliver andheat sink 20. Heat is then applied and the sliver is brazed to the heatsink 20.

The preferred method of using diamond for pad 40 is through vacuumdeposition of minute diamond particles onto heat sink 20. Thiseliminates one material-to-material interface, allowing greater thermalconductivity between the semiconductor and the heat sink, across pad 40.This also reduces the size of pad 40. As previously mentioned, a diamondsliver is usually 3 to 4 mils (0.003"-0.004") thick, but when vacuumdeposited, the diamond layer is preferably 10μ (0.00039") and generallybetween 0.3 and 0.5 mils (0.0003"-0.0005") thick. Whether a diamondsliver or diamond coating is used, the diamond pad 40 is thenpreliminarily plated with an appropriate metal and then gold plated.Semiconductor 50 is then mounted on the gold plating by applying heatand pressure. The gold plating and silicon semiconductor combine to forma gold-silicon euctectic which holds semiconductor 50 in place.

If pad 40 is made from boron nitride, aluminium nitride or alumina, itmay also be applied to heat sink 20 in two different forms: as a thinlayer; or as a thin sliver. A sliver of any of these materials would beapproximately 8-12 mils (0.008"-0.012") thick. Use of such a sliverwould necessitate the use of a preform to braze the sliver (pad 40) toheat sink 20. These materials may also be applied by plasma deposition.. .Such a deposition technique cannot be used with diamond since itinvolves heating the material until plasma forms. In the case ofdiamond, it is not likely that the cooling carbon would reform into adiamond lattice structure..!.

Plasma deposition produces a layer of material about 7-8 mils(0.007"-0.008") thick. Plasma deposition requires no preform to securethe coating. Whichever form of boron nitride, aluminium nitride oralumina is used, it must be plated with layers of molybdenum, nickel andgold. Semiconductor 50 is then secured to the gold plating as describedabove.

Using the techniques described above, the overall thickness of the RFpackage can be decreased by one-third from about 0.120" to about 0.080".However, if necessary, a Kovar spacer may be included between heat sink20 and lamina 70 to maintain an overall package thickness of 0.120".

The invention has been described as an RF transistor package andmounting pad in the best mode known to applicant, however, it will beapparent that the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theappended claims.

I claim:
 1. A semiconductor package having an input, output and a common terminal for connection therewith of respective terminals of a semiconductor which comprises:a support member; a thermally conducting, electrically insulating pad deposited directly on said support member as a thin film without use of an intermediate material between said pad and said support member, said pad adapted to receive said semiconductor; a semiconductor mounted on said pad; and a protective lid, said pad comprising . .a material selected from the group consisting of plasma deposited aluminum nitride,.!. vacuum deposited diamond. ., plasma deposited alumina, and plasma deposited boron nitride.!.. . .
 2. The package of claim 1 wherein said material is plasma deposited boron nitride..!.. .3. The package of claim 2 wherein said pad is between 7 and 8 mils thick..!.. .4. The package of claim 1 wherein said material is plasma deposited aluminum nitride..!.. .5. The package of claim 4 wherein said pad is between 7 and 8 mils thick..!.. .6. The package of claim 1 wherein said material is vacuum deposited diamond..!.7. The package of claim . .6.!. .Iadd.1 .Iaddend.wherein said pad is between 0.3 and 0.5 mils thick. . .8. The package of claim 1 wherein said material is plasma deposited alumina..!.. .9. The package of claim 8 wherein said pad is between 7 and 8 mils thick..!. 